scripts/validate-commits: use direct /bin/bash interpreter

Use of '#!/bin/bash' vs '#!/usr/bin/env bash' can be argued multiple ways.
For example, see:
https://stackoverflow.com/questions/21612980/why-is-usr-bin-env-bash-superior-to-bin-bash/21613039

It seems to be more common to use the first approach. And we don't have a
specific reason to prefer the latter. Moreover, other scripts in this repo,
like 'make-release' and 'whitespacecleanup.sh' use the former too.

scripts/validate-commits

@@ -1,64 +1,64 b''
-#!/usr/bin/env bash
+#!/bin/bash
 # Validate the specified commits against test suite and other checks.
 
 if [ -n "$VIRTUAL_ENV" ]; then
     echo "Please run this script from outside a virtualenv."
     exit 1
 fi
 
 if ! hg update --check -q .; then
     echo "Working dir is not clean, please commit/revert changes first."
     exit 1
 fi
 
 venv=$(mktemp -d kallithea-validatecommits-env-XXXXXX)
 resultfile=$(mktemp kallithea-validatecommits-result-XXXXXX)
 echo > "$resultfile"
 
 cleanup()
 {
     rm -rf /tmp/kallithea-test*
     rm -rf "$venv"
 }
 finish()
 {
     cleanup
     # print (possibly intermediate) results
     cat "$resultfile"
     rm "$resultfile"
 }
 trap finish EXIT
 
 for rev in $(hg log -r "$1" -T '{node}\n'); do
     hg log -r "$rev"
     hg update "$rev"
 
     cleanup
     virtualenv -p "$(command -v python2)" "$venv"
     source "$venv/bin/activate"
     pip install --upgrade pip setuptools
     pip install -e . -r dev_requirements.txt python-ldap python-pam
 
     # run-all-cleanup
     scripts/run-all-cleanup
     if ! hg update --check -q .; then
         echo "run-all-cleanup did not give clean results!"
         result="NOK"
         hg diff
         hg revert -a
     else
         result=" OK"
     fi
     echo "$result: $rev (run-all-cleanup)" >> "$resultfile"
 
     # pytest
     if py.test; then
         result=" OK"
     else
         result="NOK"
     fi
     echo "$result: $rev (pytest)" >> "$resultfile"
 
     deactivate
     echo
 done